Any “cowboy” .38 Special ammo is unlikely to cause leading. Leading is cased by trying to drive a cast bullet too fast, not just using cast bullets. Regular cleaning should be enough.
Black Hills “cowboy” .38 Special ammo runs at 800 FPS. That's close to regular, as in not Cowboy, Bullseye Max loads for a 158. Less than Alliant's "Cowboy" Bullseye loads.
Mind you, you have no idea what has been done to a 95 year old revolver. So stop up one end of the barrel with a rubber cork and fill the barrel with regular solvent and leave it for an hour or two. Then use a regular brush and clean it as per normal. Repeat if required.
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